Description

Use the coordinate (coord.x, coord.y, coord.z)
to do an element lookup in the 3D image object specified by
image. coord.w is ignored.

The samplerless read image functions behave exactly as the corresponding read
image functions that take integer coordinates and a sampler with filter mode
set to CLK_FILTER_NEAREST, normalized coordinates set
to CLK_NORMALIZED_COORDS_FALSE and addressing mode to
CLK_ADDRESS_NONE.

read_imageh returns half floating-point values in the range
[0.0... 1.0] for image objects created with image_channel_data_type
set to one of the predefined packed formats or CL_UNORM_INT8
or CL_UNORM_INT16.

read_imageh returns half floating-point values in the range
[-1.0... 1.0] for image objects created with image_channel_data_type
set to CL_SNORM_INT8, or CL_SNORM_INT16.

read_imageh returns half floating-point values for
image objects created with image_channel_data_type set to
CL_HALF_FLOAT.

The read_imageh calls that take integer coordinates must
use a sampler with filter mode set to CLK_FILTER_NEAREST,
normalized coordinates set to CLK_NORMALIZED_COORDS_FALSE
and addressing mode set to CLK_ADDRESS_CLAMP_TO_EDGE,
CLK_ADDRESS_CLAMP or CLK_ADDRESS_NONE;
otherwise the values returned are undefined.

Values returned by read_imageh for image objects with
image_channel_data_type values not specified in the description
above are undefined.

Notes

General information about the read_image* functions follows.

The built-in functions defined in this section
can only be used with image memory objects. An image memory object can be accessed by
specific function calls that read from and/or write to specific locations in the image.

Image memory objects that are being read by a kernel should be declared with the __read_only
qualifier. write_image calls to image memory objects declared with the
__read_only
qualifier will generate a compilation error. Image memory objects that
are being written to by a kernel should be declared with the __write_only qualifier.
read_image calls to image memory objects declared with the __write_only
qualifier will generate a compilation error. read_image and
write_image calls to the same image memory object in a kernel are
not supported.

The read_image calls returns a four component floating-point, integer or
unsigned integer color value. The color values returned by read_image
are identified as x, y, z, w where x refers to the red component,
y refers to the green component, z refers to the blue component
and w refers to the alpha component.

The following table describes the mapping
of the number of channels of an image element to the appropriate components in the
float4, int4 or uint4 vector data type for
the color values returned by read_image{f|i|ui} or supplied
to write_image{f|i|ui}. The unmapped components will be set
to 0.0 for red, green and blue channels and will be set to
1.0 for the alpha channel.

Channel Order

float4, int4 or uint4 components of channel data

CL_R, CL_Rx

(r, 0.0, 0.0, 1.0)

CL_A

(0.0, 0.0, 0.0, a)

CL_RG, CL_RGx

(r, g, 0.0, 1.0)

CL_RA

(r, 0.0, 0.0, a)

CL_RGB, CL_RGBx

(r, g, b, 1.0)

CL_RGBA, CL_BGRA, CL_ARGB

(r, g, b, a)

CL_INTENSITY

(I, I, I, I)

CL_LUMINANCE

(L, L, L, 1.0)

A kernel that uses a sampler with the CL_ADDRESS_CLAMP addressing
mode with multiple images may result in additional samplers being used internally
by an implementation. If the same sampler is used with multiple images called via
read_image{f|i|ui}, then it is possible that an implementation may
need to allocate an additional sampler to handle the different border color values that
may be needed depending on the image formats being used. These implementation allocated
samplers will count against the maximum sampler values supported by the device and given by
CL_DEVICE_MAX_SAMPLERS. Enqueuing a kernel that requires more samplers
than the implementation can support will result in a CL_OUT_OF_RESOURCES
error being returned.